Sorry, I may have misunderstood your post. I thought you were saying that, in effect, "One of you is guilty of speeding so we might as well assume it's the driver".
It's never sat easy with me that the registered keeper does not have the right to silence (despite never having fallen foul of it...
Firstly, consider a solicitor is good advice.
Having said that...
If you were genuinely unaware that the case was listed (and they do have to tell you) then you can go to court and make a statutory declaration under oath. If the magistrates accept your declaration (I heard dozens and we didn't...